Using lime and clay renders and plasters

Traditional Lime Plasters and Renders Training Day

Course overview

This practical, hands-on training day introduces the materials, principles and techniques used to prepare and apply traditional lime plasters and renders. It is designed for builders, plasterers, conservation professionals, contractors, property owners, and anyone involved in repairing or maintaining historic masonry and traditionally constructed buildings.

The course combines classroom instruction, material demonstrations and supervised practical work. It focuses on compatible, breathable finishes that can accommodate the movement and moisture behaviour of older buildings without trapping damp or damaging historic brick, stone, cob or lath backgrounds.

Participants learn how to assess an existing surface, select suitable materials, prepare backgrounds, mix and apply coats, finish the surface and provide suitable curing and protection.

The course covers internal lime and gypsum plaster, external lime render, and limewash.

Main aims

By the end of the day, participants should be able to:

- Explain the differences between lime plaster, lime render, historic and modern gypsum plaster and cement render.

- Distinguish between non-hydraulic lime, hydraulic lime and pre-formulated lime products.

- Identify suitable aggregates and understand their influence on performance and appearance.

- Assess the condition and suitability of plaster and render backgrounds.

- Prepare masonry, brick, stone, cob and lath backgrounds correctly.

- Mix and apply scratch, floating and finishing coats.

- Understand the principles of harling and thrown lime render.

- Apply appropriate finishes without overworking the surface.

- Recognise the importance of curing and weather protection.

- Identify common causes of cracking, debonding, shrinkage and failure.

- Work safely when handling lime and carrying out plastering or rendering.

Programme

Participants arrive, sign in and receive the programme and course materials. The trainer introduces the practical facilities, confirms the required protective equipment and explains the site-specific health and safety arrangements.

Traditional buildings and breathable finishes

The day begins with an introduction to traditional wall construction and the role of lime plaster and render.

Topics include:

- Solid masonry, cob, timber frame and lath backgrounds.

- How traditional walls absorb, release and redistribute moisture.

- The function of internal plaster and external render.

- The difference between protection and waterproofing.

- Why dense cement renders and impervious modern coatings can cause problems.

- The importance of compatibility between background, plaster and finish.

- The principle that the outer finish should generally be sacrificial and maintainable.

The trainer explains that “breathable” does not mean that a wall should be allowed to remain permanently wet. Good detailing, sound rainwater goods, appropriate ground levels and adequate ventilation remain essential.

Lime binders and plaster materials

The trainer, Dr Mark Womersley, introduces the materials used in traditional plastering and rendering.

Air lime

Air lime sets mainly through carbonation. It is commonly used as lime putty or hydrated lime and may suit internal plastering and sheltered external work when properly protected and cured.

Natural hydraulic lime

Natural hydraulic lime contains reactive compounds that allow it to set partly through hydration. NHL 2, NHL 3.5 and NHL 5 have different characteristics and should be selected according to the background, exposure and application.

A stronger lime is not necessarily a better lime. Excessively strong or rigid material may damage a weaker historic background.

Insulating lime renders and plasters, with the need for increased energy efficiency and breathability, can suit some conservation work.

Aggregates

Aggregates control much of the texture, colour, shrinkage, and workability of the plaster or render. The session examines:

- Sharp and well-graded sands.

- Coarse and fine aggregates.

- Local sands and colour matching.

- Aggregate particle size and coat thickness.

- The effects of excessive fines.

- Brick dust, stone dust and other additions where historically appropriate.

- Fibres such as animal hair, where required for traditional plaster systems.

- Lightweight aggregates for suitable internal applications.

The trainer demonstrates how the aggregate grading should relate to the coat being applied. A coarse backing or scratch coat generally requires a more open and robust mix than a fine finishing coat.

 Assessing backgrounds and existing finishes

Before applying plaster or render, the background must be inspected and understood.

Participants learn to assess:

- The type and condition of the substrate.

- Whether existing coatings are lime-based, cementitious, gypsum-based or synthetic.

- Soundness, suction and cleanliness.

- Cracks, hollows, salt contamination and dampness.

- Movement joints and changes in materials.

- Masonry pointing and surface irregularities.

- The condition of lath, reed matting or other plaster supports.

- Areas requiring local repair before re-plastering.

The trainer demonstrates methods of identifying hollow or detached finishes and explains when coatings should be retained, repaired or carefully removed.

The group discusses the importance of investigating the cause of failure. Persistent damp, leaking gutters, defective flashings, high external ground levels, condensation or structural movement should not be concealed beneath a new plaster or render system.

Background preparation

The trainer demonstrates preparation methods for different substrates.

Masonry backgrounds

Preparation may include:

1. Removing loose, friable or incompatible material.

2. Raking out defective joints where necessary.

3. Brushing away dust and debris.

4. Repairing large voids and unstable areas.

5. Dampening the background before application.

6. Applying a compatible spatterdash or bonding coat where appropriate.

Lath and timber backgrounds

The session covers:

- Checking the condition and spacing of laths.

- Securing loose laths.

- Replacing decayed material.

- Ensuring adequate key.

- Avoiding excessive wetting of timber.

- Understanding movement between timber and plaster.

Lunch is included

Mixing lime plaster and render

Participants take part in a supervised mixing demonstration and practical exercise.

The trainer covers:

- Safe handling of lime products.

- Accurate measurement by volume.

- Selecting the binder-to-aggregate ratio.

- Dry blending materials before adding water.

- Introducing water gradually.

- Mixing to a cohesive, workable consistency.

- Adjusting the mix for suction, temperature and aggregate moisture.

- Avoiding excessively wet mixes.

- Understanding the effect of overworking.

- Cleaning tools and equipment between mixes.

Indicative ratios may be discussed, such as approximately 1:2.5 or 1:3 binder to aggregate for some NHL render applications. However, participants are reminded that ratios must be confirmed against the specific product, aggregate and intended coat.

For traditional internal plaster, hair or other fibres may be incorporated where the historic system and coat thickness require reinforcement. Fibres must be clean, evenly distributed and compatible with the mix.

Practical application of plaster and render coats

Participants apply lime plaster or render to sample panels or a suitable training wall.

The exercise demonstrates a multi-coat system.

Scratch or backing coat

The first coat is applied to provide a sound, keyed base. It should be:

- Well bonded to the background.

- Sufficiently substantial to level minor irregularities.

- Left with an open, keyed texture.

- Protected from rapid drying.

- Allowed to stiffen adequately before the next coat.

The trainer demonstrates the use of a comb, scratch tool or suitable textured finish to create a mechanical key without excessively damaging the coat.

Floating or levelling coat

The floating coat is used to build thickness and establish the plane of the surface. Participants learn to:

 - Apply the coat without trapping voids.

- Maintain appropriate thickness.

- Avoid excessive compression or polishing.

- Rule or straighten the surface where required.

- Leave sufficient texture for the finishing coat.

- Allow the coat to firm up before further work.

Finishing coat

The finishing coat is applied more thinly using a finer aggregate. It may be finished smooth, lightly textured or in another historically appropriate manner.

Participants practise:

- Controlling suction.

- Maintaining an even thickness.

- Avoiding over-trowelling.

- Closing the surface at the correct stage.

- Blending edges and stopping points.

- Producing a consistent texture without removing the character of the material.

The number and thickness of coats depend on the substrate, specification and finish required. The trainer explains that applying one thick coat is not normally a substitute for a properly designed multi-coat system.

External renders and decorative finishes

This session focuses on external applications and traditional finishing techniques.

Render application

The trainer demonstrates how to apply external render in suitable lifts and how to keep the surface open enough to receive subsequent coats.

Topics include:

- Managing suction and dampening.

- Building up thickness gradually.

- Forming edges and junctions.

- Protecting corners and vulnerable details.

- Avoiding render bridging damp-proof courses or ventilation openings.

- Detailing around windows, doors, plinths and rainwater goods.

- Allowing adequate movement and drying time between coats.

 Surface finishes

 The course may demonstrate:

- Lightly brushed finishes.

- Floated finishes.

- Trowelled finishes.

- Textured or roughcast finishes.

- Traditional harled textures.

- Fine lime plaster finishes for internal work.

The chosen finish should be based on the building’s age, regional character and conservation requirements.

Curing, protection and review

The final session addresses curing and protection, followed by a review of the day.

Fresh lime plaster and render should be protected from:

- Direct sunlight.

- Strong wind and draughts.

- Heavy rain.

- Frost.

- Excessive heat.

- Rapid drying.

- Impact and vibration.

- Premature decoration.

Light misting, damp hessian, breathable sheeting or temporary shelter may be required. Protection must prevent rapid moisture loss while allowing sufficient air movement for carbonation and drying. The correct approach depends on the binder, coat, weather and exposure.

The trainer explains that drying and curing are not the same process. A surface may appear dry while the material beneath is still developing strength.

Common defects and their causes

 Participants examine examples of failed plaster and render, including:

- Shrinkage cracking.

- Map cracking.

- Debonding and hollow areas.

- Blistering.

- Powdering.

- Surface friability.

- Uneven colour.

- Differential drying.

- Salt crystallisation.

- Frost damage.

- Poorly bonded patches.

- Excessive trowel polishing.

- Render cracking at material junctions.

- Failure caused by cement-rich backgrounds or coatings.

The group considers whether each defect is related to:

- Poor background preparation.

- An unsuitable mix.

- Excessive coat thickness.

- Inadequate curing.

- Rapid drying.

- Excessive suction.

- Movement in the building.

- Persistent damp.

- Poor workmanship or detailing.

Learning outcome

At the end of the training day, participants will have a practical understanding of how traditional lime plasters and renders are selected, prepared, applied and cured. They will be able to assess a basic background, prepare a suitable surface, mix an appropriate material and apply a simple lime plaster or render system under supervision.

The course does not replace a project-specific specification, conservation assessment or structural investigation. It provides a practical foundation for carrying out compatible lime plaster and render work and for recognising when specialist advice is required.

All on site at our training location at a 17th-century property in Northorwam Halifax, West Yorkshire.